Unless otherwise indicated herein, the description in this section is not prior art to the claims in this application and is not admitted to be prior art by inclusion in this section.
Some typical electrophotographic-method image forming apparatuses use a two-component-development-method developer. The two-component-development-method developer is made of magnetic powder, which is referred to as a magnetic carrier, and toner, which is colorant. At a development process when forming an image, since only the toner is consumed, the toner is supplied to the developer, and then the magnetic powder and the toner are stirred. Thus, in order to replenish the toner with an amount consumed at the development process, a toner density detecting sensor, which measures toner density in the developer, is equipped with an image forming apparatus. At a color image forming apparatus, for example, since cyan, magenta, yellow, and black toners are used for development, four toner density detecting sensors are used to detect respective remaining amounts of four color toners.